Bt toxin: how it kills the pestBt विष: कीट को कैसे मारता है

Class 12 Biology · Chapter 10 · interactive diagram

Parts and their functions

• Bacillus thuringiensis (Bt) (बैसिलस थुरिनजिएंसिस (Bt)) — A soil bacterium. Some strains make proteins that kill certain insects — lepidopterans (tobacco budworm, armyworm), coleopterans (beetles) and dipterans (flies, mosquitoes).
• Protoxin crystals (inactive) (निष्क्रिय प्राकविष क्रिस्टल) — The bacterium forms protein crystals in one phase of its growth. The toxin is stored as an INACTIVE protoxin — that is why it does not kill the bacterium itself.
• Bt cotton (Bt कपास) — Specific Bt toxin (cry) genes were isolated from the bacterium and put into cotton. The plant now makes the protoxin itself — a bio-pesticide, so less chemical insecticide is needed.
• Insect larva (pest) (कीट का लार्वा) — When a pest such as the cotton bollworm feeds on the Bt plant, it swallows the inactive protoxin.
• Midgut (alkaline pH) (मध्यांत्र (क्षारीय pH)) — The alkaline pH of the insect gut dissolves (solubilises) the crystals and converts the protoxin into the ACTIVE toxin.
• Active toxin (सक्रिय विष) — The activated toxin binds to the surface of midgut epithelial cells.
• Midgut epithelial cells (मध्यांत्र उपकला कोशिकाएँ) — Cells lining the insect midgut — the target of the active Bt toxin.
• Pores in the cells (कोशिकाओं में छिद्र) — The bound toxin creates pores in the epithelial cells.
• Swelling and lysis → death (फूलना व लयन → मृत्यु) — Through the pores the cells swell and burst (lysis); the insect eventually dies.
• cry genes (cry जीन) — The toxin is coded by genes named cry. Most Bt toxins are insect-group specific: cryIAc and cryIIAb proteins control cotton bollworms; cryIAb controls the corn borer.

Steps

1. Bacillus thuringiensis makes protein crystals of an INACTIVE protoxin — so it does not harm itself.
2. A specific cry gene is cloned into the crop (e.g. Bt cotton); the plant now makes the protoxin.
3. An insect larva feeds on the plant and swallows the protoxin crystals.
4. The alkaline pH of its midgut dissolves the crystals → ACTIVE toxin.
5. The active toxin binds to midgut epithelial cells and makes pores.
6. The cells swell and lyse (burst) — the insect dies.

Original diagram drawn by Simple Learning Hub, based on the NCERT textbook. Report a mistake